有個開發寫了不帶引數的OVER()分析函式，類似如下：

SELECT categories_products.category_id,
  COUNT(*) OVER() AS all_products
FROM categories_products;

經查，其用途相當於不帶GROUP BY子句的聚合操作，只不過所有行都包含該結果。分析函式本質性就像標量列的行為一樣，可以和聚合函式一起使用。

SELECT DISTINCT
  categories.category_id,
  categories.name,
  COUNT(categories_products.product_id) OVER
 
(PARTITION BY categories_products.category_id) AS products,
  COUNT(categories_products.product_id) OVER() AS all_products
FROM categories
INNER JOIN categories_products
ON categories.category_id = categories_products.category_id

就帶PARTITIONBY子句的分析函式而言，其效能通常遠比單獨計算好。那不帶選項的OVER()呢？

findptdis=# explain analyze select
 
 sjno_id,count(1) over() from sjno_local1;
                                                          QUERY PLAN                                                          
------------------------------------------------------------------------------------------------------------------------------
 WindowAgg  (cost=
 
0.00..142400.00 rows=640000 width=32) (actual time=779.573..995.977 rows=2560000 loops=1)
   ->  Seq Scan on sjno_local1  (cost=0.00..134400.00 rows=640000 width=24) (actual time=0.009..359.622 rows=2560000 loops=1)
 Planning Time: 0.047 ms
 Execution Time: 1055.498 ms
(4 rows)

findptdis=# explain analyze select sjno_id,x from sjno_local1,(select count(1) x from sjno_local1) f;
                                                                   QUERY PLAN                                                                    
-------------------------------------------------------------------------------------------------------------------------------------------------
 Nested Loop  (cost=136000.00..276800.02 rows=640000 width=32) (actual time=376.336..833.559 rows=2560000 loops=1)
   ->  Aggregate  (cost=136000.00..136000.01 rows=1 width=8) (actual time=376.325..376.327 rows=1 loops=1)
         ->  Seq Scan on sjno_local1 sjno_local1_1  (cost=0.00..134400.00 rows=640000 width=0) (actual time=0.006..302.627 rows=2560000 loops=1)
   ->  Seq Scan on sjno_local1  (cost=0.00..134400.00 rows=640000 width=24) (actual time=0.007..300.224 rows=2560000 loops=1)
 Planning Time: 0.105 ms
 Execution Time: 879.682 ms
(6 rows)

可見OVER()的效能是不如單獨計算的。

https://www.mendelowski.com/docs/postgresql/aggregate-functions-over-partition/